When hemoglobin or dry blood is heated with a few drops of glacial acetic acid, and, if necessary, a small crystal of NaCl, there are formed yellowish, microscopic crystals called hemin, or Teichmann's crystals. These crystals of hemin, which is the hydrochloride of heme, are very characteristic of blood and, therefore, furnish us with a reliable and delicate test for blood. However, this test does not enable us to differentiate between the blood of man and that of the lower animals.